The present invention relates to drill bits and methods of forming drill bits, and more particularly, to methods and molds for reducing the amount of inclusions present in drill bits when formed.
In traditional casting processes, a furnace melts a metal, and the molten metal is then transferred, typically to another vessel, such as, for example, a ladle, to transport it to the forming means, such as, a mold. The furnace can also act as a holding location for the molten metal to allow any trapped gases and low density impurities to migrate to the surface. A typical mold used in a casting process may include a number of components for forming a metallic piece. For example, a common gating system can comprise a pouring cup connected to a downsprue, which feeds liquid metals to runners and runner extensions that feed into the final mold shape. As used herein, “casting” and “casting processes” refer to this traditional process in which metal is melted at one location and transferred to a second forming means, such as a mold.
In the casting of metals, it is generally desirable to separate exogenous (i.e., originating from sources external to the melt) intermetallic and non-metallic inclusions from the molten metal. Such inclusions can result, in molten metals, from impurities included in the raw materials used to form the melt, from slag, dross and oxides which form on the surface of the melt as a result of reactions with atmospheric gases such as oxygen, and from small fragments of the refractory materials that are used to form the chamber or vessel in which the molten metal melt is formed. Such inclusions, if not removed from the molten state of the metal, can result in weakened points in the final formed and solidified metal body, which is the eventual downstream end product of the melting operation.
Typically, in a metal casting process, the molten metal is formed in a furnace wherein the constituent components are added in the form of unmelted scrap and/or refined virgin metal, deoxidizing agents in various forms (both solid and gaseous or a combination of both) and alloying elements. Very light (less dense) solids and gases tend to migrate to the surface of the melt where they either effervesce or float in combination with partially and completely solidified oxides known variously as slag and dross. Slag can be skimmed off of the molten metal before the molten metal is removed from the furnace, removing a majority of the impurities trapped in the slag. Further, a second slag can form in the ladle and be skimmed off for additional inclusion removal prior to pouring the molten metal into the casting. The higher density impurities in the melt tend to remain in some degree of suspension in the liquid phase of the metal, or melt, as the fluid flow convection currents are generated within that melt by the heating means applied by the melting furnace. Flux can be used to chemically bind with some inclusions, allowing them to float or otherwise be removed from the liquid phase. Those inclusions which are chemically inert may not be affected by the flux and may therefore remain suspended in the liquid phase of the metal. In the casting process, filters have also been used to remove larger inclusions from the molten metal as it passes into the mold. In such processes, the pressure provided by the head of the downsprue or some additional pressurization unit may be used to provide the driving force necessary to pass the liquid metal through a filter and into the mold.
For the purposes of this invention, casting should be distinguished from infiltration (a means of forming a metal object wherein a molten metal binder is wicked into contact with a powder present in the mold) processes. Typical casting processes can involve multiple operations that allow for multiple locations and steps during which inclusions can be removed. Conversely, operations involving infiltration are more single-step in orientation, and therefore, do not typically include more than one step during which the inclusions could be removed. This makes using a mechanical means to remove inclusions more difficult in infiltration processes.